Specification for FWaaS changes for DVR

Spec addresses the changes required for FWaaS to work with
DVR to handle:
* Perimeter Firewall support on N - S traffic
* Ensure that E - W DVR traffic is not broken

Implements: blueprint neutron-dvr-fwaas

Change-Id: I2ea7e39b03655cf8ff5fe533b50c5d33ade6720f
Merge "Specification for FWaaS changes for DVR"